The wall seems to be at the exact length that makes that impossible, I'm not sure though. The solution I had in mind for 12 builders is very hard to execute, so I probably won't try much more. Here is the solution:
Set the release rate to about 80 before any lemmings come out. After one is out, set it back to the minumum. Make #1 build as close to the edge as possible. When #2 is as close to the edge of #1's first step as possible, make him build. Make #3 build two or three pixels away from the next edge. After that, the lemmings should be sealed up so they won't fall off the screen. Make #4 (actually could be any lemming, I'm just calling him #4) build to the right on #3's lowest step. Make #4 build two more times so that he hits his head on the steel, leaving a one pixel gap obove his last step. When he is low enough to get onto the next platform, make him build to the left. Set the release rate to 99. Make #4 build two more times to get onto the next platform, then do it again to get to the exit. Make any other lemming build to seal up the one pixel gap.